The present invention relates to an information playback method and apparatus which play back a signal recorded on an information recording medium and, more particularly, to an information playback method and apparatus which play back a recorded signal by using partial response equalization and Viterbi decoding.
As a technique of increasing the recording density by signal processing, the PRML (Partial Response Maximum Likelihood) technique including Viterbi decoding processing is generally known. According to the PRML scheme, a playback signal is equalized to a PR (Partial Response) waveform having intentionally given symbol interference, and data is discriminated by a Viterbi detector using the multilevel transition caused by intersymbol interference.
FIG. 13 shows the playback circuit unit of a conventional information playback apparatus which discriminates data by the PRML scheme. After an analog playback signal obtained from an information recording medium 110 is input to a low-pass filter 111, the output from the low-pass filter 111 is digitized by an AD converter 112. The output from the AD converter 112 is input to a PLL (Phased Locked Loop) circuit 113 to obtain a time-series playback data string Yd (d is an integer of one or more) sampled at a clock period. The time-series playback data string Yd is input to an FIR (Finite Impulse Response) filter 114 to be equalized to a predetermined partial response waveform Ht (t is an integer satisfying 1≦t<d) by convolution computation with a tap coefficient Ws (s is an integer satisfying 1≦s<d) of the FIR filter 114. The playback data after partial response equalization is input to a Viterbi detector 118, which in turn outputs identification data Ad. The identification data Ad is input to an FIR filter 115 to calculate an ideal signal (target waveform) by convolution computation with the partial response waveform Ht. This ideal signal is input to a coefficient calculator 119. The equalized playback data is also input from the FIR filter 114 to the coefficient calculator 119.
Techniques for partial response equalization include several techniques such as a zero force method and MMSE (Minimum Mean Square Error) method. In general, the MMSE method is often used. As disclosed in Japanese Patent Laid-Open No. 2000-123487 (reference 1), the MMSE method is a technique of determining a tap coefficient Ws so as to minimize the mean square value of d of an equalization error vd defined by
                              v          d                =                                            ∑              s                        ⁢                                          Y                                  d                  -                  s                                            ×                              W                s                                              -                                    ∑              t                        ⁢                                          A                                  d                  -                  t                                            ×                              H                t                                                                        (        1        )            
More specifically, therefore, the tap coefficient Ws is determined to minimize ε′ expressed by
                              ɛ          ′                =                              E            ⁡                          [                                                (                                                                                    ∑                        s                                            ⁢                                                                        Y                                                      d                            -                            s                                                                          ×                                                  W                          s                                                                                      -                                                                  ∑                        t                                            ⁢                                                                        A                                                      d                            -                            t                                                                          ×                                                  H                          t                                                                                                      )                                2                            ]                                =                      E            ⁡                          [                              v                d                2                            ]                                                          (        2        )            
where E[ ] represents the operation of obtaining the mean value of d in the expression in [ ], and the expression in [ ] represents the square of the equalization error vd defined by equation (1), i.e., the square of the difference between the equalized playback data and the ideal signal. Referring to FIG. 13, the coefficient calculator 119 receives the equalized playback data string from the FIR filter 114 and the data string of the ideal signal from the FIR filter 115, and determines the tap coefficient Ws for the FIR filter 114 so as to minimize the mean square value of d of the equalization error vd, i.e., ε′.
More specifically, techniques of obtaining a condition for minimizing ε′ or a quasi-condition for the minimization include, for example, a technique of obtaining a condition (equation) for nullifying the deviation of the tap coefficient Ws by matrix calculation, and a technique of repeatedly updating and obtaining the tap coefficient Ws starting from a proper initial value of the tap coefficient Ws. The latter technique is disclosed in Japanese Patent Laid-Open No. 2001-189053 (reference 2) and the like.
As disclosed in Japanese Patent Laid-Open No. 2003-303417 (reference 3) and the like, a technique of calculating the partial response waveform Ht by using the least squares method based on a playback waveform from a disk is known.
The present inventors, however, have found that equalization based on the MMSE method does not necessarily minimize the data detection error (bit error rate: to be referred to as “bER” hereinafter).
This is because the decoding performance of the Viterbi detector 118 varies depending on whether the noise characteristic of a signal input to the Viterbi detector 118 is chromatic or white. A general definition about whether noise is white or chromatic will be described below by using the above equalization error vd. Assume that with respect to the noise component (equalization error) vd, an autocorrelation Rf defined by Rf=E[vd×vd+f] takes a non-zero finite value only for R0, and takes 0 for all values other than R0. In this case, this noise is white. If there are noise components which take non-zero finite values for values other than R0, e.g., R1 and R2, the noise is chromatic. If the noise characteristic of data input to the Viterbi detector 118 is white, the Viterbi detector 118 can exhibit optimal detection performance. In general, however, since the noise characteristic of a playback signal obtained from the information recording medium 110 is chromatic, the bER cannot be generally minimized by equalization based on the MMSE method.